<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
        let arr=[1,2,3,4,5,6,7];
        arr.reduce((previousValue,currentValue,currentIndex)=>{
            alert(`第${currentIndex}次，${previousValue}+${currentValue}`);
            return previousValue+currentValue;
        });
    </script>
</body>
</html>